1. Field of the Invention
The present invention relates to a ribbon microphone circuit including an automatic switching system on the circuit configuration.
2. Related Background Art
A ribbon microphone, a type of dynamic microphone, can produce output without power sources. Since the output level from the ribbon microphone unit is low, a step-up transformer is typically connected to the output terminals of the unit. The ribbon microphone unit is connected to a primary winding of the transformer, and output terminals are provided across a secondary winding thereof. The ribbon microphone unit can output signals at a practical level.
The step-up transformer boosts the output level of the ribbon microphone unit by N times, where N represents a winding ratio of the step-up transformer (1/N=the number of primary winding/the number of secondary winding). That is, the output level increases with an increase in the winding ratio N. The step-up transformer, however, causes an increase in output impedance of the ribbon microphone unit. The output impedance of the ribbon microphone unit increases in proportion to the square (N2) of the winding ratio of the step-up transformer. This may result in large output impedance that causes external noise to be easily mixed.
In a ribbon microphone unit with an output impedance of 600Ω and an output level of −52 dB/Pa, for example, if a winding ratio of the step-up transformer is increased so that the output level is increased to −42 dB/Pa, the output impedance increases to 5.4 kΩ. Such a large output impedance causes output signals to easily pick up noise during transmission through a cable, as generally known. The industrial standard of microphones thus defines an output impedance value to be 1 kΩ or less. It is therefore desirable to provide a circuit configuration that can increase the output level while the output impedance is substantially maintained at a normal output level.
To reduce the output impedance, a buffer amplifier circuit including an emitter-follower circuit is connected to the subsequent stage of the step-up transformer. The buffer amplifier circuit, however, requires a power source, and thus a circuit for supplying external power is further necessary to incorporate the buffer amplifier circuit. A known power source connectable to a microphone is a phantom power supply. It is thus demanded to provide a ribbon microphone circuit including an automatic switching system such that, at an increased output level, the buffer amplifier circuit and an external power supply circuit for operation of the buffer amplifier circuit are connected to the ribbon microphone circuit while, at a normal output level, the microphone circuit is operated without connection of the buffer amplifier circuit.
Specifically, it is demanded to provide a ribbon microphone circuit including an automatic switching system such that the microphone outputs signals having a low impedance and a high level while the phantom power supply is being connected, and outputs signals having a high impedance (about 600Ω) and a low level (about −52 dB/Pa) while the phantom power supply is being not connected.
In other words, it is demanded to provide a ribbon microphone circuit including an automatic switching system such that the output impedance can be maintained at a low level by a special circuit at an increased boost ratio of the step-up transformer for an increased output level, and the special circuit can be automatically disconnected at an ordinary output level that is compatible with an output impedance in accordance with the industrial standard. Although there is no existence of related art documents for the purpose of solving the above described problems, Japanese Unexamined Patent Application Publication No. 2007-312260 relating the present application is provided in this specification.
Japanese Unexamined Patent Application Publication No. 2007-312260 discloses a microphone circuit including an amplifier circuit and a power supply circuit for supplying external power, the power supply circuit being automatically switchable to allow a microphone to produce an output even if external power is not supplied. Specifically, the microphone circuit includes an amplifier circuit for amplifying electrical signals output from an electroacoustic transducer module; an external power supply circuit that supplies external power for driving the amplifier circuit; a light that is turned on while the external power is being supplied through the external power supply circuit; and contacts that can be turned on or off depending on the state of the light,. Such components are connected such that the amplifier circuit output signals as a microphone output during a lighting period of the light, while the electroacoustic transducer module outputs signals as a microphone output during a non-lighting period.